This invention relates to a method and apparatus for the manufacture of concrete and concrete-like products. While the invention is directed toward the manufacture of a wide and diverse range of products such as panels, including terrazzo panels, concrete blocks, cinder blocks, ceramic products, bricks, adobe constructions, concrete pipe, drainage tile, silo block or staves, pre-cast tile, filler base, pre-stressed products, and other load-bearing and non-load-bearing (curtain wall) structures, all of various sizes and shapes, the invention is most particularly directed to the manufacture of concrete and concrete-like structures that are very large in size, and intricate in configuration, such that they do not lend themselves toward conventional manufacturing techniques.
In prior art types of manufacture of concrete and concrete-like products, cement, aggregate, additives if any, and water are generally blended in various ratios. Usually, a sufficient amount of water is added to completely hydrate the cement and also to impart a degree of fluidity (substantial liquidity) to the blended mixture, to form what is often referred to as a wet mix. The blended mixture is then poured or compacted into a form and allowed to take an initial set. The initial set usually occurs within one-half hour to four hours, depending upon the temperature and mixture employed. After forming this initial set, the article usually has very little strength, and in order to increase its strength, the article is generally allowed to cure for a period of time from several days, to several weeks or more.
This excessively long period of time required for both the initial set, and for the development of its ultimate strength causes considerable problems in the manufacture of cast concrete products. Principally, the concrete mixture must remain in the mold until the product acquires sufficient strength to be self-sustaining, so that it will not fall apart when the mold is removed. This is especially so if the products or articles are to be large in size, and intricate in configuration. Furthermore, the concrete product, even after the mold is removed must be allowed to cure for an extended period of time before it is subjected to any substantial stresses. The need to maintain the concrete products in the mold until the initial set is obtained, and for curing the products until the ultimate strength is developed requires a substantial inventory of molds, and a large storage area. Also, the high costs involved in handling weak products in their initial stages of cure, in accordance with prior art processing techniques is often quite costly in both man power and storage space. Often, of even greater consequence is the cost in terms of number of rejects, or of imperfections in the products, especially as their sizes and intricacy in configuration increases. Such rejects are generally the result of imperfections due to internal stresses and even fractures that result from handling. However, such imperfections may be due to differential shrinkage characteristics at different locations of the articles or products, for example.
Attempts have been made to decrease the cure time in the manufacture of concrete and like products. Some such attempts have included the various types of accelerators that have been added to the concrete mixtures, as well as heating and steaming processes, for use during manufacture. These methods at best have produced marginal improvements with respect to reducing the curing time and have been somewhat expensive. For example, in high pressure steam curing the product often had to be steamed for twelve to twenty-four hours, in order to obtain a satisfactory hardness and strength in the final product. In addition certain of the processes had adverse effects upon the composition. High pressure steaming of concrete products containing substantial amount of marble or limestone, for example, can cause a very marked decrease in the ultimate compressive strength of the final product. Accordingly, for some compositions, such techniques are not desirable.
In order to increase the strength of concrete products, additives have previously been suggested. One patent of significance, because it teaches the use of a vinyl acetate-dibutyl maleate copolymer for improving compressive strength of articles is the Murray U.S. Pat. No. 3,305,506. It should be noted, however, that while the disclosure thereof is one that results in a beneficial effect upon improving the compressive strength of the product, the ultimate compressive strength of the product is still relatively low, compared with products made in accordance with the present invention. Furthermore, the use of such an additive is desirable with respect to the handling of smaller, preferably flat panels. However, in the manufacture of very large panels, for example, 4 Ft. by 8 Ft., and several inches, or even a foot or more in thickness, having intricate shapes, including ribs, cross ribs, etc., such as might be used in the formation of building structures, such as walls or the like, it is not possible for such an additive to give the structure sufficient strength that will enable it to stand alone, unsupported, when removed from the mold, or in most instances, even to be removed from the mold.
Some processes have been developed for curing articles produced in a mold, generally by removing the mold from the apparatus and storing it in a room for a given period of time, and wherein the article is subjected to steam and/or carbon dioxide gas within the room, for example at an elevated pressure which can facilitate the formation of a skin or surface cure thereon. However, the formation of such a cure can in itself inhibit deeper curing, by precluding further passage inward, to a deeper location, of carbon dioxide gas, for example.